Night vision imaging systems (NVIS), such as night vision goggles, allow for image generation under extreme low light conditions. For example, a pilot may use night vision goggles to view a simulated scene (a “stimulated simulation”). Infrared (IR) projectors may generate an IR image corresponding to a conventional visible-light image; the IR image may be visible only through the night vision goggles. However, the IR projectors may not generate sufficient energy to simulate light halos: large, bright circular areas surrounding points of bright light. The NVIS goggles may be externally stimulated; for example, the IR image generator may render light points and halos so as to be visible only in the IR stream, e.g., as polygons approximating a round halo.
However, as these rendered polygonal halos are a screen-space effect, if a light point (around which the halo is to be generated, with the light point as a center) is visible, the complete halo must also be visible. For example, a halo may disappear from the field of view only when the whole halo itself, rather than its associated light point, moves off an edge of the field of view. If a light point is occulted (e.g., obscured, temporarily or permanently, by another object situated between the light point and the observer) the halo must be made to disappear from view. If the image generator simply rendered a halo for every light point, occultation errors may occur where halos persist when their associated light points are occulted. This may not be an issue for runway modeling, where the points of light will not be occulted during simulated landing operations. To accurately render halos for visible points of light under non-runway conditions an occultation test (e.g., line-of-sight) must be performed for every halo candidate point (an expensive process to maintain under real-time conditions).